1. Field of the Invention
The present invention is related to a radiation CT apparatus. More specifically, the present invention is related to a radiation CT apparatus that obtains radiation images of subjects by sequential imaging.
2. Description of the Related Art
There are known bed type radiation CT apparatuses which are employed to perform radiation imaging of breasts (refer to U.S. Patent Application Publication Nos. 20060094950 and 20070064867). In these radiation CT (Computed Tomography) apparatuses, a patient lays face down on a support base, and the patient's breast is fitted through an opening which is formed in the support base. Radiation imaging is performed with the breast which is positioned below the support base as the subject.
The above radiation CT apparatuses are equipped with a radiation source that emits radiation, and a detecting panel that detects the emitted radiation. When performing radiation imaging, the radiation source and the detecting panel are provided such that they face each other with a breast positioned below a bed interposed therebetween. Radiation imaging of the breast is sequentially performed while integrally rotating the radiation source and the detecting panel about an axis of rotation that passes vertically through the breast. Thereby, a plurality of radiation images of the breast are obtained, which are thereafter reconstituted to generate a radiation CT image.
When radiation is irradiated onto the detecting panel during radiation imaging, electrical image signals that represent a radiation image of the subject are accumulated in each detection pixel that constitutes the detecting panel. When reading out the radiation image recorded in the detecting panel, image signals are read out from all of the detection pixels of the detecting panel. For this reason, sequential radiation imaging is performed by these radiation CT apparatuses such that a radiation imaging operation is performed after all of the image signals which were accumulated by a preceding radiation imaging operation have been read out from all of the detection pixels of the detecting panel.
In addition, the distance from the axis of rotation to the radiation source and the distance from the axis of rotation to the detecting panel are fixed in the above radiation CT apparatuses. That is, these radiation CT apparatuses are configured such that the imaging magnification rate of the radiation images of the subject, which are recorded in the detecting panel, is constant.
Breasts, which are the subjects of radiation images, vary in size. Radiation CT apparatuses are equipped with detecting panels of a size that enables radiation imaging of extremely large breasts. In the case that radiation imaging of a smaller breast is performed using these detecting panels which are large enough to image extremely large breasts, the ratio of the area of the detecting panel in which a radiation image of the breast is formed becomes small. That is, the image signals recorded in the detecting panel by radiation, which is irradiated thereon during radiation imaging of the breast, include a large amount of unnecessary image signals of regions, so called blank pixel regions, in which radiation has not passed through the breast.
However, readout of image signals from detecting panels is performed by reading out image signals from all of the detection pixels that constitute the detecting panels. That is, effective image signals that represent an image of the breast and unnecessary image signals that represent the blank pixel portions are read out without being distinguished from each other. For this reason, the ratio of unnecessary image signals which are read out from the detecting panel when radiation imaging of a smaller breast is performed. The utilization efficiency of the detecting panel deteriorates compared to a case in which radiation imaging of a large breast is performed. In addition, the readout efficiency of image signals also deteriorates.
In the case that radiation images of a breast are obtained sequentially, a single detecting panel is used to perform imaging and readout repeatedly. Therefore, the effect of deteriorated readout efficiency becomes great.
Note that this problem is not limited to cases in which breasts are imaged, but is common to all types of radiation imaging by radiation CT apparatuses.